1. Field of the Invention
The invention relates to brake shims and more particularly to lubricating systems for shims.
2. Description of the Related Art
To reduce noise and vibration in modern brake systems, it has become increasingly common to use shims. In its simplest form, a shim is a thin sheet of metal which is clipped, riveted or adhered to the back of a backing plate away from its friction side. The shim acts as an intermediate element (or buffer) between the backing plate and piston of the braking system. The piston contacts the shim instead of the backing plate directly. The shim takes on a portion of the force, thereby preventing noise and vibration when the brake is applied.
Some shim systems use tabs on the edges to retain the shim to the backing plate. This is believed to be particularly advantageous over fixed shims as the clips allow the shim to move slightly against the backing plate. However, the metal shim scraping against the metal backing plate can itself cause noise and vibration.
To remedy this, some shim manufacturers have resorted to using rubber, cork, ceramic or other materials for sound-dampening and to reduce heat transfer that can result from the friction. Others use a greasy lubricant between the shim and the backing plate. The lubricant must be applied by hand when the brake pad is installed. The technician squeezes lubricant onto the shim and smears to cover before clipping the shim onto the back of the brake pad. To apply fresh lubricant, the shim must be disassembled from the pad and then re-clipped into place after the lubricant has been applied. Over time, taking off and putting on the shim tends to weaken the side tabs. They can become less springy, or deform or even break off entirely. Since the tabs frictionally grip the edges of the backing plate, removal and re-insertion of the shim also means scraping along the edges of the backing plate which can cause wear over time.
Depending on its viscosity, the lubricant has a tendency to either sit in one spot on the shim, or drain or leak out of the space between the shim and the plate, in each case resulting in loss of efficacy of the lubricant. To address this, some manufacturers have developed complicated multi-shim arrangements—an inner shim with a cut out section encased in or overlapped by an outer (non-cut out) shim. The cut out sections on the inner shim are intended to act as reservoirs, each holding a small amount of the lubricant. Such an arrangement is not optimum due to the added manufacturing costs of the second shim.
It would be desirable to address or ameliorate the foregoing problems by providing a simple single tab-style shim that includes a compartment for lubricant, and the compartment having one or more access ports to allow lubricant to be inserted while the shim is in place. This would permit maintenance without removal, so that the life of the shim can be extended.